The ARN flight lands just before a bank of EU flights land in T5, so if they are early, you should be OK, if not, then even deplaning quickly won't help if you end up behind a 747 from BA/LH filled with Chinese/Indians that have visas and will be grilled by the CBP. The luggage is usually on the belt by the time you are past immigration, make sure you just drop the bag on the belt just before the exit and don't queue for the UA checkin on the right.

If you miss the connection and have to overnite, ask for the Hilton opposite the terminal, if they don't give it, go back to T5 and ask for a SAS station rep: if they bring you in late, the carrier who got you into ORD has to pay to reaccomodate. They can rebook you onto AA or other carriers...@:-)

If all fails, the Hyatt at the airport has a 65$/night distressed traveller rate. The United agents can give you a paper to show the hotel that you are eligible.